1. Field of the Invention
The field of the invention relates to the manufacture of ammonia. More particularly, the field of the invention relates to a novel continuous process for the recovery of ammonia from the purge gases of an ammonia manufacturing plant in the form of an aqueous solution, with the concentration of water in the solution controlled within a narrow range that permits the solution to be blended into the main anhydrous ammonia product of the plant to give a water content in the product which is high enough to provide corrosion protection to carbon steel product storage equipment, but which is still low enough to avoid exceeding the maximum specification for water in the shipped product.
2. Description of the Related Art
Many processes are known for the manufacture of ammonia. A number of such processes are described in the book titled "Ammonia", edited by A. V. Slack and G. R. James, published by Marcel Dekker, 1977. A typical process, the Chemico ammonia process, is shown in Part III, page 303, FIG. 3 of that reference. In the Chemico process, as in most commercial processes, ammonia is synthesized from a mixture of hydrogen and nitrogen at an elevated pressure and temperature in an ammonia converter. Because of equilibrium limitations, only part of the hydrogen and nitrogen are converted to ammonia in a single pass through the converter. The gas leaving the converter is cooled in a series of heat exchangers to temperatures at which most but not all of the ammonia formed is liquified and then separated from the unconverted gas in the primary and secondary separators. The unconverted gas is combined with fresh makeup feed gas containing hydrogen and nitrogen and then recycled to the converter.
Makeup gas normally contains inert gaseous impurities, such as methane and argon, which do not react. Such inerts tend to accumulate in the circulating gas, reducing the partial pressures of the hydrogen and nitrogen, thereby suppressing the conversion of hydrogen and nitrogen to ammonia. To limit the concentration of inerts in the circulating gas to a tolerable level, some of the recycle gas is withdrawn as a purge stream. In the Chemico process cited above purge gas is withdrawn from the circulating gas stream leaving the primary separator.
In the Chemico process the purge gas is used for fuel in the primary reformer. Because purge gas withdrawn from a recycle stream contains a residual amount of ammonia vapor not condensed from the reactor effluent, it is desireable to recover that ammonia vapor as a valuable increment of production. In the Chemico process part of the ammonia in the purge gas is recovered by chilling the gas with ammonia refrigerant to a still lower temperature, at which additional ammonia is condensed and then separated from the purge gas in another separator.
Liquid ammonia separated from converter effluent contains a small amount of dissolved gases. When the liquid is reduced in pressure from reaction pressure to a lower storage pressure, as in the Chemico process, most of the dissolved gases flash out of the liquid into a gaseous phase. This gas also contains a small amount of ammonia vapor which it is desireable to recover. Such flash gas is normally combined with the purge gas as in the Chemico process. As used herein, the term purge gas refers to any gaseous stream or streams withdrawn from ammonia synthesis reactor effluent streams, before or after combining with fresh makeup feed gas and before or after any cooling, liquefaction, compression, pressure reduction, or vapor-liquid separation step.
Two principal means have been used to increase the recovery of ammonia from purge gas. One is to cool or refrigerate the purge gas to a lower temperature, at which additional ammonia is condensed and separated, as in the Chemico process. One disadvantage of this method is that the additional refrigeration requires additional investment and consumes additional energy, increasing costs, and it still leaves part of the ammonia in the purge gas. Another disadvantage, when the purge gas is to be used as fuel, is that the residual ammonia tends to increase the level of environmentally objectionable nitric oxides in the combustion gases discharged to the atmosphere.
In some ammonia manufacturing processes the purge gas is further processed in a cryogenic or membrane separation system to separate and recover the hydrogen and nitrogen for recycle, or to recover other valuable components such as argon. In such cases any residual ammonia must be completely removed ahead of the cryogenic or membrane system, so that refrigeration alone is not an acceptable means of recovering the ammonia such cases.
The other principal means for increasing the recovery of ammonia from purge gas is to scrub the gas with water, which absorbs the ammonia to produce an aqueous solution of ammonia. Disadvantages of the water absorption method as practiced heretofore are that the ammonia solution that is recovered contains too much water to blend back into the anhydrous ammonia product, and is of value only if there is a local use for such a solution, as in an ammonium nitrate plant.
If there is no such use, then an additional system to fractionate the solution to recover the contained ammonia in anhydrous form is normally employed, increasing investment costs and energy cost. Such a system is described in an article titled "25 years of purge gas recovery", by W. H. Isalski, published in "Nitrogen", No. 152, pages 100-105, and shown in FIGS. 2 and 3 of the article.
With or without fractionation of the aqueous solution, the concentration of ammonia in the aqueous solution has been limited to a level at which the aqua can be stored at atmospheric pressure when cooled to a temperature which can be achieved with ordinary cooling water. Such a concentration level generally falls in the range of 20 to 25 weight percent.
A heretofore unrelated factor is that anhydrous ammonia which contains no water at all is known to be more corrosive to carbon steel storage equipment than ammonia which contains a small concentration of water, greater than 0.1%, preferably about 0.2 wt %. This factor has been unrelated to ammonia recovery from purge gas, because when the refrigeration method is used for recovery of ammonia from purge gas, no water is present in the recovered ammonia. And when a conventional water scrubbing method is used, the amount of water in the recovered ammonia is far greater than that required for corrosion protection, and far greater than that allowed by most commercial specifications for ammonia product, ranging from 0.1 to 1.0 wt % water and typically in the range 0.2 to 0.5 wt % water.
When ammonia is absorbed in water, heat is evolved, heating the solution to a temperature higher than that of the water used for absorption. Higher temperatures reduce the concentration of ammonia that can be reached with a given ammonia partial pressure in the incoming gas. To increase the concentration of ammonia in the aqueous solution, heat is sometimes removed by indirect heat exchange with cooling water within the absorber or between two or more absorption stages.
The use of a refrigerant in combination with water scrubbing has heretofore been avoided because of the extra costs mentioned above, because aqueous solutions of higher concentrations than can be achieved with cooling water cannot be stored at atmospheric pressure, and because of the risk of chilling the water or dilute solution below its freezing point somewhere in the absorption system, causing plugging and shutdown.
The need exists, therefore, for a continuous process for recovering ammonia from ammonia plant purge gases which provides for essentially complete recovery, which provides a water concentration in the total plant product which meets the requirements of both corrosion protection and product specifications, which avoids any freezing or plugging in the recovery system, and which provides all of these efficiently and at minimum investment and operating costs.